In recent years, research of electronic devices such as organic thin film transistors and the like receive attention, and various materials to be used for the electronic devices are investigated. Among these, development of materials for forming an insulating layer included in the electronic device is actively carried out in order to suppress deterioration of the electronic device over time.
The “electronic device” means an active device which uses actions of electrons. The “electronic device insulating layer” means an insulating layer included in an electronic device, and examples thereof include an organic thin film transistor insulating layer, an organic electroluminescence device insulating layer, an electronic paper insulating layer, an RFID tag insulating layer, and a liquid crystal display insulating layer. An electronic device insulating layer material means a material used for forming the above-mentioned electronic device insulating layer. Hereinafter, the present invention will be explained by using organic thin film transistors as a representative example of the electronic device.
Since an organic thin film transistor which is one aspect of the electronic device can be produced at lower temperatures than inorganic semiconductors, a plastic substrate or film can be used as a substrate of the organic thin film transistor, and by using such a substrate, a device which is flexible, and is lightweight and is hardly breakable can be obtained. Moreover, since there are cases where a device can be produced by film formation using a method of applying or printing a solution containing an organic material, there are also cases where a large number of devices can be produced on a substrate of large area at low cost.
Furthermore, since there are a wide variety of materials which can be used for the investigation of organic thin film transistors, a device with a wide range of varied characteristics can be produced by using materials varying in molecular structure in the investigation.
In electric field effect type organic thin film transistors, which are one type of organic thin film transistors, the voltage applied to a gate electrode acts on an organic semiconductor layer through a gate insulating layer, and thereby the current amount of a drain current is controlled. In addition, a gate insulating layer formed between a gate electrode and an organic semiconductor layer prevents a drain current from flowing to the gate electrode.
Moreover, organic semiconductor compounds to be used for the formation of organic semiconductor layers of electric field effect type organic thin film transistors are susceptible to environmental influences such as humidity, oxygen and the like, and therefore transistor characteristics are likely to be deteriorated over time due to humidity, oxygen and the like.
Therefore, in the device architecture of a bottom-gate type organic thin film transistor, which is one kind of electric field effect type organic thin film transistors, with an organic semiconductor compound exposed thereon, it is necessary to form an overcoat insulating layer covering the whole structure of the device so as to protect the organic semiconductor compound from being in contact with the open air. On the other hand, in the device architecture of a top gate type organic electric field effect transistor, which is another kind of electric field effect type organic thin film transistors, an organic semiconductor compound is coated and protected with a gate insulating layer. Thus, insulating layer materials are used in order to form a gate insulating layer and an overcoat insulating layer covering organic thin film transistors, both of which cover the organic semiconductor layer.
In the specification, an insulating layer or an insulating film of an electronic device such as the above-mentioned gate insulating layer and the overcoat insulating layer is referred to as an electronic device insulating layer. A material used for the formation of an electronic device insulating layer is referred to as an electronic device insulating layer material. Also, a material used to form the organic thin film transistor insulating layer is referred to as an organic thin film transistor insulating layer material.
The organic thin film transistor insulating layer material is required to have electrical insulating properties and characteristics superior in electrical breakdown strength when having been formed into a thin film. Further, particularly in the bottom-gate type electric field effect type organic thin film transistor, an organic semiconductor layer is formed on the gate insulating layer. Therefore, the organic thin film transistor gate insulating layer material is required to have affinity with an organic semiconductor compound for forming an interface in close contact with the organic semiconductor layer and to have flatness of the surface on the organic semiconductor layer side of the film formed from the organic thin film transistor gate insulating layer material.
As a technology responding to such requirements, Patent Document 1 describes an organic thin film transistor insulating layer material in which an epoxy resin is used in combination with a silane coupling agent and an organic thin film transistor formed by using the organic thin film transistor gate insulating layer material. In this technology, a hydroxyl group produced at the time of a curing reaction of the epoxy resin is reacted with the silane coupling agent. The reason for this is that the above-mentioned hydroxyl group enhances the hygroscopic properties of the organic thin film transistor insulating layer material and impairs the stability of transistor performance.
Non-Patent Document 1 describes the use of a resin prepared by thermally cross-linking polyvinylphenol and a melamine compound at 175° C. for a gate insulating layer. In this technology, by cross-linking with the melamine compound, the hydroxyl groups contained in the polyvinylphenol are removed and the film strength is increased simultaneously. A pentacene TFT having this gate insulating layer has low hysteresis and exhibits durability to a gate bias stress.
Non-Patent Document 2 describes that polyvinylphenol and a copolymer prepared by copolymerizing vinylphenol with methyl methacrylate are heated at 150° C. and used for a gate insulating layer. In this technology, the polarity of the whole film is reduced by interactions between the hydroxyl group of vinylphenol and the carbonyl group of methyl methacrylate. A pentacene TFT having this gate insulating layer has low hysteresis and exhibits stable electric properties.
However, in consideration of practical use such as driving a light emitting device such as an organic electroluminescence device (organic EL device), the operating accuracy of an organic thin film transistor has to be further improved and the fall effect of a hysteresis is still insufficient in the above-mentioned conventional gate insulating layer.
In addition, in consideration of practical use of an organic thin film transistor, it is necessary to form a penetrating part such as a via hole in an insulating layer formed between an upper electrode and a lower gate electrode and to join the upper electrode and the lower gate electrode.
However, since the above-mentioned conventional material does not have photosensitivity, it is difficult to be patterned at the time of formation of an insulating layer. Therefore, to form a penetrating part such as a via hole in an insulating layer, it requires the steps of: firstly applying a solution containing a resist material on an insulating layer and forming a resist layer; exposing the resist layer through a mask; developing the resist layer and to form a pattern of the resist layer; transferring the pattern to the insulating layer by using the resist layer as a mask in which the pattern was formed; and carrying out the exfoliation and the removal of the resist layer in which the pattern was formed. Thus, the production process becomes complicated.
A metal is usually used for an electrode of an organic thin film transistor, and a metal electrode is formed above an organic layer. A metal electrode is usually formed by forming a metal layer on the whole surface of an organic layer by the sputtering process and by the subsequent patterning. By passing through this process, a member which has many devices on a substrate of a large area can be simply produced.
However, since a metal steam used for the sputtering process has high energy, characteristics of an organic compound in an organic layer may vary when the organic layer contacts the metal steam.
Particularly, in a step of etching and a step of the liftoff at the time of patterning a metal layer, an etching solution containing a relatively strong alkali or acid is used. Such a relatively strong alkali or acid contained in an etching solution may change characteristics of an organic compound in an organic layer of a base.
If characteristics of an organic compound in an organic layer vary by a metal steam, strong acid, or strong alkali, the surface of the organic layer exposed by the patterning at the time of the formation of an electrode changes in characteristics as compared with the condition before the formation of an electrode and the performance of an organic thin film transistor is adversely affected. For example, if an organic insulating material is used as a gate insulating layer of an organic thin film transistor and then a metal is vapor-deposited directly on an insulating layer by the sputtering process to form a metal layer, and the metal layer is patterned to form a sauce electrode and a drain electrode, the hydrophilicized surface of the gate insulating layer is exposed and transistor characteristics are diminished.
Patent Document 2 describes an organic thin film transistor in which a solution containing a polytitanometalloxane and 1-butanol is applied to the whole surface of the gate insulating layer composed of an organic compound; and a barrier layer with a high solvent resistance is formed. The gate insulating layer is protected by the barrier layer from a metal steam used for the formation of a metal layer, an etching liquid used in the patterning of a metal layer, an organic solvent used in the formation of an organic semiconductor layer and the like.
However, a polytitanometalloxane is highly chemically stable. Accordingly, in order to etch a polytitanometalloxane layer, it is necessary to use very strong alkali solution. Thus, if an alkali solution contacts an underlying organic layer, the surface of an organic layer is damaged.